Light emitting diodes (LEDs) are very well known in solid state illuminating devices, commonly used for illuminators, display components and indication devices. Early LEDs provided radiation in the high frequency end of the visible spectrum, namely as red light. More recently, LEDs have been developed that operate efficiently in the higher frequency portion of the visible spectrum, namely to emit blue light. Such devices employ layers of luminescent or fluorescent materials, for example phosphor, overlying the LED die and serving to shift the wavelength of the light emitted from the die, the radiation being re-emitted from the intermediate phosphor layer at a longer wavelength.
As an example, a blue LED such as a gallium nitride (GaN) die, can be used to provide a white LED lamp by positioning a layer of phosphor dye over the LED die. This is then encapsulated in a standard clear epoxy lens, and the LED then connected and powered in the conventional manner. The phosphor layer serves to absorb, shift, and re-emit a broad band yellow-green light, as well as some unabsorbed original blue light which passes through the phosphor layer. The combination radiation is emitted through the epoxy lens and results to a viewer in a perceived emission of white light.
Various types of fluorescent material have been tested and used in providing a range of colour mixtures, and devices are now available that can produce light in almost any desired colour.
A further approach has been developed in the prior art for employing such fluorescent or luminescent materials in LED devices. In U.S. Pat. No. 5,847,507 (Hewlett Packard Co.), a process and apparatus is described in which a lens incorporating a fluorescent dye is over-moulded to a short wavelength light emitter such as a blue LED or a laser diode placed within a reflector cup. The concentration of the dye within the lens can be varied to control the extent of a region within the lens where the majority of the radiation is re-emitted, such that the remaining portion of the lens can fulfil its primary duty of focussing the light.
The dye used for “doping” the epoxy material in constructing this type of LED device can be provided by way of an inorganic dye material, generally a powder, or by way of an organic dye material, generally a liquid. Studies by the inventor of the present invention have indicated that the use of organic dyes appears to be more efficient than that of inorganic compounds, largely because the powder particles tend to settle within the hardening epoxy material, so tending to affect the homogeneity of the phosphor-containing portion of the lens. These studies have shown that the LED devices so produced can therefore suffer from problems of non-uniformity in their colour. Portions of the radiation pattern of the LED as perceived can differ in colour from other portions, and in particular, the outer part of the radiation pattern can tend to differ from the colour of the on-axis radiation emitted.
As an example, in the construction of an LED device consisting of a blue LED with YAG:Ce3+ phosphor, a yellow ring can be seen around the perimeter of the radiation pattern. Therefore, viewing the lamp on-axis and off-axis can result in perception of different colours. The reason for this artefact has been found to be that the quantity of luminescent or fluorescent material surrounding the LED die tends to be non-uniform, due to the method of construction of such devices. The doped epoxy material is conventionally applied over the LED die in a quantity such that it fills the reflector cup in which the die is mounted. The phosphor particles contained in the epoxy material then settle around the contours of the LED die and reflector cup as the epoxy cures. Due to the contours of the device, and particularly due to the generally annular space around the LED die, excessive dye material tends to collect at the sides of the LED, in comparison with that which remains overlying the surface of the LED. For this reason, the wavelength shift of the light emitted from the device when seen from the side differs from that when viewed along a line parallel to the central axis.
It is an object of the present invention to address the aforementioned drawbacks and to provide an improved apparatus and method in the field of light emitting devices.